Neisseria meningitidis is a leading cause of bacterial
meningitis and sepsis in the United States. N. meningitidis is
classified into serogroups based on the antigenic characteristics
of its capsular polysaccharide. During 1989-1991 in the United
States, serogroups B and C accounted for most (91%) of invasive
meningococcal disease while serogroup Y caused less than 5% (1);
however, during 1992-1995, serogroup Y accounted for an increasing
proportion of meningococcal disease. This report describes the
epidemiology of serogroup Y meningococcal disease (SYMD) during
1991-1996 in Illinois and Connecticut, which conducted enhanced
surveillance through active reviews of clinical records, and in
areas participating in active laboratory-based surveillance during
1989-1995. The findings indicate a substantial increase in the
proportion of meningococcal disease caused by N. meningitidis
serogroup Y since 1989.

Illinois

In Illinois (1990 population: 11,430,602), 589 cases of
invasive disease attributed to N. meningitidis were reported from
January 1991 through March 1996, representing an annual incidence
ranging from 0.9 to 1.0 per 100,000 population. Serogrouping was
conducted for 371 (83%) of 447 culture-confirmed cases. The
proportion of SYMD increased from 6% in 1991 to 29% in 1995; the
proportion of disease attributed to serogroups B and C decreased
from 85% to 59%.

From January 1991 through March 1996, the Chicago Department
of Public Health received 145 reports of suspected meningococcal
disease among persons residing in Chicago (1990 population:
2,783,726), and N. meningitidis was isolated from a normally
sterile site in 133 (92%) case-patients. The overall annual
incidence of culture-confirmed disease ranged from 0.7 to 1.3 cases
per 100,000 population. Of the 105 culture-confirmed isolates for
which serogroup was known, 42 (40%) were serogroup Y; 29 (28%),
serogroup B; 27 (26%), serogroup C; and two (2%), serogroup W-135.
Among case-patients with known serogroups, the proportion of SYMD
increased from 6% in 1991 to 71% in 1995. In comparison, the
proportion of serogroups B and C decreased from 94% to 25%. Of 42
patients in Chicago with culture-confirmed SYMD, 22 (52%) were
female; two (5%) died. The median age of patients with SYMD was 16
years compared with 2 years for patients with disease caused by
non-serogroup Y meningococci. Although patients with SYMD were more
likely to present with purulent sputum (six {14%} versus one {2%})
and chest pain (eight {19%} versus one {2%}), they were not more
likely to have an infiltrate on chest radiograph (seven {23%}
versus 10 {20%}). Case-fatality rates were similar among patients
with SYMD (two {5%} of 42), compared with case-patients with
disease caused by other known serogroups (six {10%} of 63).

Connecticut

From January 1991 through June 1996, a total of 190
culture-confirmed cases of invasive N. meningitidis infection among
residents of Connecticut (1990 population: 3,287,116) were reported
to the Connecticut Department of Public Health. The overall annual
incidence of culture-confirmed cases ranged from 0.7 to 1.4 per
100,000 population. Of the 144 isolates for which serogroup was
known, 69 (48%) were serogroup C; 38 (26%), serogroup Y; 35 (24%),
serogroup B; and two (1%), serogroup W-135. The proportion of SYMD
increased from 1991 (6%) to 1995 (35%). Of the 33 case-patients
with SYMD identified since 1994, 18 (55%) were female; two (6%)
died. The median age for patients with SYMD was 29 years, compared
with 13 years for patients with disease caused by non-SYMD.

Active Laboratory-Based Surveillance

During 1989-1995, active laboratory-based surveillance was
conducted in three counties in the San Francisco metropolitan area,
eight counties in the Atlanta metropolitan area, and four counties
in Tennessee, and during 1992-1995, in Maryland, representing an
aggregate population of approximately 12 million. A case was
defined as N. meningitidis isolated from a normally sterile site in
a resident of a surveillance area. In the three active surveillance
areas for which continuous data were available, the rate of SYMD
per 100,000 persons increased from 1989 (0) to 1995 (0.4). Among
the case-patients for whom serogroup was known, the proportion of
SYMD increased from 1989 (0) to 1995 (32.5%). During the same
period, the overall rate of meningococcal disease remained stable
at 1.0-1.4.

During 1992-1995, in the four active surveillance areas, SYMD
patients were older than patients with non-serogroup Y (median age:
21.8 years versus 14.2 years). Pneumonia was four times more likely
to be diagnosed in persons with SYMD (12%) than in persons with
other serogroups (3%), even after adjusting for age.

Laboratory Investigation

Multilocus enzyme electrophoresis (MEE) (2) using 24 enzymes
was used to characterize genetic relatedness of serogroup Y
isolates systematically sampled from 1995 Illinois surveillance
(n=40), 1995 Connecticut surveillance (n=17), 1992-1995 U.S. active
laboratory-based surveillance (n=40), 1972-1975 U.S. active
laboratory-based surveillance (when SYMD accounted for 18% of the
isolates submitted to CDC) (n=27) (3), and 1970-1974 surveillance
of U.S. military personnel (n=12). Two major enzyme type complexes
could be distinguished by a difference in peptidase mobility. One
group of enzyme types accounted for 54% (33 of 97) of the isolates
tested during 1992-1995, one of the 1972-1975 surveillance strains,
and none of the strains from U.S. military personnel. The other
group accounted for 34% (33 of 97) of 1992-1995 isolates and 62%
(24 of 39) of the 1972-1975 isolates.

Editorial Note

Editorial Note: During the 1970s, SYMD was recognized as a common
cause of endemic disease in some U.S. populations (3,4) and was
associated with several outbreaks in military personnel (5-7).
During 1978-1981, SYMD caused 7% of meningococcal cases reported
through a nationwide surveillance system in which 27 states
participated (8). Although SYMD accounted for only 2% of endemic
disease in U.S. active surveillance during 1989-1991 (1), by 1995
the proportion of infections caused by SYMD had increased in
Illinois and Connecticut and in the active surveillance areas. In
1995, among the 30 states reporting supplemental data on
culture-confirmed cases of meningococcal disease through the
National Electronic Telecommunications System for Surveillance
(NETSS) (9), serogroup information was recorded for 527 (54%) of
973 cases reported, and serogroup Y accounted for 21% of cases.
This pattern is consistent with the findings in this report and
underscores the need to both determine and report serogroup
information for all cases of meningococcal disease.

The finding in this report that patients with SYMD in Chicago,
Connecticut, and the active laboratory-based surveillance areas
were older than patients with disease caused by non-serogroup Y
meningococci is consistent with cases reported to CDC through
NETSS. One possible explanation for this and the increase in SYMD
is waning population immunity against SYMD. However, the increase
in SYMD also may reflect, in part, the emergence of a distinct
clone that differs in peptidase motility, as characterized by MEE.
Although the association between epidemic meningococcal disease and
clonality has been clearly established, the possible relation
between shifts in endemic disease serogroup distribution and
emergence of particular clones requires further assessment.

The clinical illness associated with SYMD differs from that of
the other serogroups; in particular, findings from the active
laboratory-based surveillance system indicated that pneumonia was
more common among patients with SYMD, consistent with studies in
some military populations (5,6) in which serogroup Y was more
likely than other serogroups to be associated with pneumonia and
other forms of nonmeningitic disease. Meningococcal pneumonia may
not be diagnosed because isolation of the organism from the sputum
cannot distinguish persons who are meningococcal carriers from
those with pneumonia caused by this organism, and because
physicians may not consider N. meningitidis as a possible cause of
pneumonia. As a result, infections that occur in the absence of
meningitis or bacteremia may be underreported in current
surveillance.

The current meningococcal vaccine (Connaught Laboratories,
Swiftwater, Pennsylvania), which contains the purified
polysaccharide capsules of serogroups A, C, W-135 and Y, has been
effective in controlling serogroup C outbreaks and may be useful in
controlling an SYMD outbreak. However, this vaccine has not been
used to control endemic disease because its immunogenicity is low
in young children and immunity is of limited duration. Conjugated
vaccines for serogroup C, which are similar to those now available
for preventing Haemophilus influenzae type b, are being evaluated
in safety and immunogenicity trials (10). Because of the increased
proportion of SYMD, manufacturers should consider developing a
serogroup Y conjugate component for controlling endemic
meningococcal disease.

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